In hydraulic shock absorbers as known, for instance, from German Offenlegungsschrift No. 2,146,222, for braking the shock on structural parts of automotive vehicles, there are small openings in the relatively thick inner pipe wall of a working cylinder, the cylinder being made of very strong material, and the openings being successively covered by a working piston upon actuation of the shock absorber, and thus determining the characteristic of the braking force. Cutting type production (e.g., boring) or these openings, which commonly are of 0.4 to 1 mm diameter, for example, is normally too expensive for series manufacture of shock absorbers in large numbers, because of considerable wear on the small boring tools, and because of the need to remove the burrs in a subsequent work step. Non-cutting production by stamping with punch and die in the customary way, wherein the punch diameter is substantially the same as the hole diameter, is out of the question if holes of the exemplary diameters previously mentioned are to be produced where the wall thickness is two to three mm, as occurs in the particular example given. Under such circumstances, the punch immediately bends and becomes unusable from the forces involved.
The problem confronted by Applicants, namely production of small openings in a relatively thick pipe wall, is solved in accordance with the instant invention in that the wall lies on a die with an opening that is the same or substantially the same as the opening that is to be produced, and a conical tip of a working tool is forcefully guided to penetrate into the wall in alignment with the die opening, the depth of penetration corresponding approximately to the wall thickness.
From German Pat. No. 958,800, a process is known for production of a small opening in a platelike object by means of a tool or mandrel. In this process, there is borne on an incompressible plastic mass a metallic intermediate plate, and then on this another metallic intermediate plate is placed, and the object to be formed with an opening is placed on the upper intermediate plate. A mandrel with a conical tip is pressed into the object so far that a through opening is produced in it by tearing. There is the drawback that there is no guarantee of freedom from burrs, or of preciseness of opening dimensions. This known process entails a conical bending of the platelike object, and would not be suitable for the purposes of the present invention, where there is to be no supplementary deformation on the opening side of the sheet or pipe wall that is to be provided with an opening, as in a shock absorber, which must remain round.
The process or method of the instant invention offers substantial cost advantages as opposed to cutting production of small holes in a thick wall. In the process, there is necessarily a conical widening of the wall opening, which is desired in the case of the shock absorber mentioned previously. In a simple way, there are produced shutterlike jet openings for the stream of liquid flowing outward from a working cylinder, the openings having a stable flow characteristic that is slightly dependent upon viscosity. Such a flow characteristic is a condition to uniform action of a shock absorber.
In the process of the invention, there first is an impression of the sheet or pipe wall with the conical tip of the tool. The tip penetrates into the wall, whereby the material is caused to flow. With further penetration of the conical tip, part of the material of the wall is forced into the die opening. With still further penetration, corresponding approximately to the thickness of the wall, the lump of material forced into the die opening is sheared off at the edge of the die opening, whereby the lump can detach from the wall before the conical tip is seated in the die opening. Hereby the actual opening is produced, its size being determined by the size of the die opening. The shearing off of the lump of material below the punch occurs surprisingly and advantageously so uniform that openings successively produced are almost identical, and without burrs. Different opening cross sections can be produced by suitable configurations of the conical tip.